Interpretive Summary: Excessive levels of toxic metals in the soils, such as aluminum (Al), cadmium (Cd) and Zn (Zn), limit crop production on extensive land areas worldwide. Considerable research is being conducted by many laboratories to investigate the molecular and physiological mechanisms of plant tolerance to these toxic metals. In this paper we discuss recent progress that has been made on mechanisms of plant Al and heavy metal tolerance. Two primary tolerance strategies plants employ are to either exclude the metals from the roots, or to tolerate the accumulation of the metals in the plant. Research on Al tolerance has identified an Al exclusion mechanism based on Al activation of organic acid exudation from the root. These organic acids move out of root cells into the soil where they bind and detoxify Al. Results from research aimed at cloning the genes conferring this mechanism of Al tolerance are discussed. Recent research on heavy metals has focused on unique plants that not only tolerate high levels of heavy metals in the soils, but also accumulate these metals to very high levels in the above-ground plant biomass. Research on the molecular basis for this extreme metal accumulation is discussed in the context of using plants to clean up contaminated soils. The long term goals of this type of research are to identify both Al tolerance genes to be used for improving crops for cultivation on Al-toxic soils, and to identify metal hyperaccumulation genes to be used to develop transgenic plants suited for the remediation of heavy metal contaminated soils.

Technical Abstract:
Plants have evolved sophisticated mechanisms to deal with toxic levels of metals in the soil. In this paper, we present an overview of recent progress with regards to understanding fundamental molecular and physiological mechanisms underlying plant resistance to both aluminum (Al) and heavy metals. The discussion of plant Al resistance will focus on recent advances in our understanding of a mechanism based on Al exclusion from the root apex, which is facilitated by Al-activated exudation of organic acids. The consideration of heavy metal resistance will deal with molecular and physiological investigations of an interesting metal hyperaccumulating plant species, the Zn/Cd hyperaccumulator, Thlaspi caerulescens.